Cassini Shows Off

Ice and Rock Mixture Inside Titan

By precisely tracking NASA's Cassini spacecraft on its low swoops over Saturn's moon Titan, scientists have determined the distribution of materials in the moon's interior. The subtle gravitational tugs they measured suggest the interior has been too cold and sluggish to split completely into separate layers of ice and rock.

This artist's illustration shows the likely interior structure of Saturn's moon Titan deduced from gravity field data collected by NASA's Cassini spacecraft. The investigation by Cassini's radio science team suggests that Titan's interior is a cool mix of ice studded with rock, though the outermost 500 kilometers (300 miles) appear to be ice essentially devoid of any rock. Many planets and moons, including the Earth, evolve into a body with a clearly distinct rocky core. This radio science investigation suggests Titan's interior, cool and sluggish, failed to allow the interior to separate into completely differentiated layers of ice and rock.

The finding shows how Titan evolved in a different fashion from inner planets such as Earth, or icy moons such as Jupiter's Ganymede, whose interiors have split into distinctive layers.

"These results are fundamental to understanding the history of moons of the outer solar system," said Cassini Project Scientist Bob Pappalardo, commenting on his colleagues' research. "We can now better understand Titan's place among the range of icy satellites in our solar system."

Scientists have known that Titan, Saturn's largest moon, is about half ice and half rock, but they needed the gravity data to figure out how the materials were distributed. It turns out Titan's interior is a sorbet of ice studded with rocks that probably never heated up beyond a relatively lukewarm temperature. Only in the outermost 300 miles is Titan's ice devoid of any rock, while ice and rock are mixed to various extents at greater depth.

"To avoid separating the ice and the rock, you must avoid heating the ice too much," said David J. Stevenson, one of the paper's co-authors. "This means that Titan was built rather slowly for a moon, in perhaps around a million years or so, back soon after the formation of the solar system."

This incomplete separation of ice and rock makes Titan less like Jupiter's moon Ganymede, where ice and rock have fully separated, and perhaps more like another Jovian moon, Callisto, which is believed to have a mixed ice and rock interior. Though the moons are all about the same size, they clearly have diverse histories.

The Cassini measurements help construct a gravity map, which may help explain why Titan has a stunted topography, since interior ice must be warm enough to flow slowly in response to the weight of heavy geologic structures, such as mountains.

Creating the gravity map required tracking minute changes in Cassini's speed along a line of sight from Earth to the spacecraft as it flew four close flybys of Titan between February 2006 and July 2008. The spacecraft took paths between about 800 to 1,200 miles above Titan.

"The ripples of Titan's gravity gently push and pull Cassini along its orbit as it passes by the moon and all these changes were accurately recorded by the ground antennas of the Deep Space Network within 5 thousandths of a millimeter per second [0.2 thousandths of an inch per second] even as the spacecraft was over a billion kilometers [more than 600 million miles] away," said Luciano Iess, the paper's lead author. "It was a tricky experiment."

The results don't speak to whether Titan has an ocean beneath the surface, but scientists say this hypothesis is very plausible and they intend to keep investigating. Detecting tides induced by Saturn, a goal of the radio science team, would provide the clearest evidence for such a hidden water layer.